Add documentation note to test selective sync

The Geo Self-service framework provides a bunch of shared tests. But to
be able to use them, the factory for the replicable you're working on
should have a `project` attribute.

doc/development/geo/framework.md

@@ -560,6 +560,10 @@ Metrics are gathered by `Geo::MetricsUpdateWorker`, persisted in
    end
    ```
 
+1. Make sure the factory also allows setting a `project` attribute. If the model
+   does not have a direct relation to a project, you can use a `transient`
+   attribute. Check out `spec/factories/merge_request_diffs.rb` for an example.
+
 Widget replication and verification metrics should now be available in the API,
 the Admin Area UI, and Prometheus!